Some things I've learned from experimenting with these Mg blocks:
1) Use the back of your knife blade to get the scrapings. Rather than running the knife along the flat edge of the block, scrape the corner of it. In this fashion, you're constantly regenerating a "peak" or corner to scrape off. When it gets a little harder to scrape, move to another corner/edge.
2) Larger curls seem to work a little better than a fine powder.
3) If you think you're going to scrape enough Mg off of the block to burn for an extended period to light kindling, you'll be disappointed. The best way to get a fire going using one of these, that I've found, is to use it to get coarser tinder (like pine needles or wood shavings) going and use that to light the kindling.
4) The best way to get the coarser tinder going, that I've found, is to scrape a pile of the Mg and position the coarser tinder just on the side of that, so that you can light the Mg and it'll flare up to light the other tinder. If the coarser tinder it covering too much of the Mg, in many instances, it creates a "fire break" in the pile of Mg and the Mg flares up and goes out without lighting the fire.
5) It doesn't take much Mg to get a fire going in this method. However, care must be taken with the Mg shavings. They must be kept in a tight pile so that it all burns. If the filings are not close enough together, part of the shavings will go up and the rest will remain unlit.
6) In a windy situation, try to block the shavings so that they're out of the wind. As many have said, they're light and tend to blow around in a strong breeze.
All in all, it's an OK way to get a fire going. I prefer the Fire Steel, but that's me. I have several of these Mg blocks and they're not bad, once I figured out the tricks. With the right positioning, you don't need to spend 10+ minutes scraping together a pile of Mg to light. I can usually get a fire going with this in only a few minutes, and most of that is getting the Mg -> coarser tinder -> kindling arranged before lighting.